The following invention relates to a humidity sensor and in particular to a calibrated humidity sensor for use in industrial or office heating and cooling systems.
Heating and cooling systems for climate control in large buildings such as office buildings and industrial plants not only regulate temperature relative to the ambient atmosphere, but also temperature as a function of relative humidity. In addition, humidifiers are sometimes used with such systems and it has been found that relative humidity in the range of 45-55% is the optimal comfort range. In addition, certain environments in process control must have their humidity regulated fairly precisely. Another example of an environment requiring regulation of relative humidity is a greenhouse which requires the maintenance of a high relative humidity.
The regulation of humidity and/or temperature in response to humidity is controlled by a humidity sensor. In the past, relative humidity sensors have been of two general types. Some humidity sensors use a humidity sensitive polymer on a porous ceramic plate. The resistivity of the polymer changes as a function of relative humidity. The problem with such devices is that they are not accurate enough for most applications and are also subject to chemical deterioration in harsh environments. Another type of humidity sensitive device employs a capacitor with an air dielectric. Since the dielectric constant of air is one and the dielectric constant of water is about 80, changes in the relative humidity between the capacitor plates changes the dielectric, and, hence, the capacitance of the sensor. The changes in capacitance can be used in a number of ways in circuits to provide an electrical output that is indicative of the relative humidity.
The problem with such devices is that they are sensitive and the nominal capacitance of such devices can be altered, for example, during installation. Systems that employ this type of capacitor, however, obviously depend upon the accuracy of the sensors' nominal capacitance. Nominal capacitance is also important because the slope of the curve describing capacitance versus relative humidity may differ for different capacitances. For example, small capacitors sold under the trade name MiniCap by Panametrics of Waltham, Mass. exhibit linear characteristics for changes in capacitance over a wide range of relative humidity. However, the linear slope of such capacitors differs for capacitors having different capacitances. In general, the higher the capacitance, the steeper the slope of the capacitance versus relative humidity curve. Thus, a difference in capacitance from the nominal rated capacitance of the sensor not only affects its nominal reading, but its reading at higher relative humidities. While quality control assures that capacitors leaving the factory meet nominal standards, by the time such units are assembled into sensors and installed in the field, the capacitance of such units may have changed. This leads to unacceptable errors in the measurement of relative humidity.